Electronic equipment in general, particularly computers, including the portable ones, such as laptops and notebooks, are typically formed by electric circuits and devices which, for a good functioning, require that their temperature be maintained within a certain temperature range, which is previously determined and mainly lower than its superior limit, in order to guarantee the operational properties thereof.
When carrying out a determined function, said electro-electronic devices transform part of the electric energy used for the operation into heat, noise, etc. This part of energy converted in thermal energy should preferably be withdrawn from the equipment, so as to allow its thermal management to be adequately carried out, providing greater levels of efficiency and reliability of the components, so that these components can operate in moderate temperature levels.
Traditionally, in said equipment, the electronic circuits are accommodated in a casing or cabinet for protection and assembly and, due to the space disposition of their components, many of the devices that generate heat are positioned in central regions that are difficult to access from the outside. In these cases, the electric device responsible for heat generation is located in the interior of the electronic equipment, and consequently, the generated thermal energy must be transferred to the external environment in which the equipment is operating. When the dissipation levels are low or moderate, it is traditionally provided finned dissipation devices positioned close to the heat source for increasing the area exposed to the air in the interior of the equipment and which is responsible for the dissipation of energy. This process may or may not be aided by forced ventilation, by installing fans in the dissipation device or in the cabinet itself. Independently of the location or of the presence of fans, the air available to absorb the heat from the dissipation device arrives at the device already previously heated by the other components of the electronic equipment, which also dissipate, in the form of heat, part of the energy used for their operation. This pre-heating reduces the efficiency of the process of transferring heat from the component desired to be cooled. In order to reduce this effect, the heat is usually efficiently conducted from the device to be cooled to a region close to the side of the equipment where the thermal energy can be absorbed by the air in the operational environment of the equipment without pre-heating. To said end, several components are provided to transport energy in an efficient manner. The known solutions for withdrawing heat from these components are based on passive devices as, for example: heat conduction in a solid medium, heat pipes, thermosiphons, liquid pumping circuits and refrigeration circuits by mechanical vapor compression (FIGS. 5 to 5d).
In all the embodiments presented, the dissipation device may or may not be housed in the interior of the equipment to be cooled. FIGS. 5 to 5d illustrate dissipation devices carried by the electronic equipment.
In the solutions presented in FIGS. 5 to 5d, the heat dissipation region is positioned in an outermost portion of the equipment, so as to facilitate removing the heat to the external environment. The efficiency in conducting the heat from its source to the environment external to the equipment can be obtained by a working fluid phase-change process, as it occurs in the solutions illustrated in FIGS. 5a, 5b and 5d, and by a high flow rate of a circulating working fluid which may or may not change phase and which is impelled by a circulation pump (FIG. 5c). Except in the mechanical vapor compression (FIG. 5d), the typically used working fluid is water. In the case of the heat-pipes (FIG. 5a) and thermosiphons (FIG. 5b), the confined working fluid is in equilibrium with a portion of the volume occupied by the fluid in the liquid phase and another portion occupied by the fluid in the gaseous phase. The liquid phase is directed to the region coupled to the heat source by gravitational orientation or by capillary effect provided by porous elements immersed in the liquid and gaseous phases, and changes phase as it removes energy from the heat source. After being vaporized, this working fluid in the gaseous phase migrates to the cold portion of the component, heat-pipe or thermosiphon, which is exposed to the external environment air, where the energy is dissipated. The heat removal causes again a phase change of the working fluid from the gaseous to the liquid phase, thus restarting the cycle.
In the fluid pumping circuit, the working fluid in the liquid phase is continuously impelled from the hot source to the cold portion of the circuit, transferring energy from the heat source to the external environment air through a high flow rate imposed by the propulsion device, usually a pump. The working fluid is then heated when in contact with the heat source and posteriorly cooled when exposed to the external environment air. While being a more efficient process than the one provided by a finned dissipation device positioned on the heat source, the fluid pumping is less efficient than the dissipation devices composed by heat-pipes and thermosiphons, once there is no phase change and the propulsion element consumes energy.
In one of the known prior art solutions which uses heat pipes (U.S. Pat. No. 7,116,552), heat removal from the heated region of the computer is carried out through a heat dissipation system presenting two refrigeration circuits of the passive type (heat-pipe), each formed by a respective heat pipe. In this construction, a first refrigeration circuit presents a first heat pipe having a first end attached to the heated region of the computer and a second end mounted in a heat exchange device which also houses a first end portion of a second heat pipe of a second refrigeration circuit, so that heat exchange between the second end of the first heat pipe and the first end portion of the second heat pipe occurs in said heat exchange device. In this construction, after heat is exchanged between the second end of the first heat pipe and the first end portion of the second heat pipe, in the heat exchange device, the heat is conducted away from the heated region of the computer, being then dissipated to the external environment.
While this construction can be applied to portable computers without compromising their available area, the heat transfer and heat dissipation which occur only by actuation of the capillary pump, are not as efficient as those obtained in the refrigeration systems using mechanical vapor compression.
In the technical solution which provides conduction of thermal energy using a refrigeration system by mechanical vapor compression, a working fluid in the gaseous phase and coming from an evaporator coupled to the heat source, is compressed in a compressor and directed to a condenser exposed to the external environment air. In this heat exchanger, called condenser, in which the working fluid in the gaseous phase returns to the liquid phase, the thermal energy is removed by the external environment air and the condensed working fluid is then directed to an expansion device responsible for reducing its pressure, so that it can be evaporated in the evaporator and, posteriorly, compressed by the compressor, completing the cycle. The fact that the working fluid presents two distinct pressures (low pressure in the evaporator and high pressure in the condenser), throughout the refrigeration cycle, allows the energy conduction process to occur with temperature variation and, thus, the electronic element can be cooled at temperature levels inferior to the ones found in any of the other alternatives, and even reach temperatures inferior to the external environment air temperature itself.
Nevertheless, the use of such refrigeration systems by mechanical vapor compression presents some barriers regarding not only the miniaturization of the compressor, but also the efficiency of the known evaporators, when their size is reduced to the usual dimensions of the processors, and the like.
Besides, in most systems available for the thermal management of electronic equipment, the cooling system carried by the equipment is dimensioned for one operational range, that is, for determined levels of energy to be removed during the operation of the equipment. For reasons of space and also of energetic consumption, such systems are not dimensioned to offer the highest efficiency during peaks of operation, since they occur in reduced time periods or with intermittent time intervals.
In the case of computers, mainly the portable ones, the cooling system is dimensioned for the normal operation of the equipment under low and moderate processing levels, as it occurs in operations of text editing, Internet browsing, image editing, and the like. When more severe levels of processing or frequency of overclocking operation are required, the cooling system does not operate satisfactorily and requires a higher refrigeration capacity.